TEHRAN | In an outpouring of people power not seen here since the 1979 Iranian revolution, tens of thousands of Iranians marched through the streets of Tehran on Monday to protest allegations that President Mahmoud Ahmadinejad had won re-election through massive fraud.
Opposition candidate Mir Hossein Mousavi, who many here think was the real victor of Friday's elections, emerged from seclusion for the first time since the vote to address the crowd, which was estimated to number as many as 1.5 million people.
Supreme Leader Ayatollah Ali Khamenei, the Muslim cleric who initially confirmed an Ahmadinejad victory, abruptly changed direction and promised a probe into allegations of ballot-rigging, although it was not clear whether the action was merely a ploy to curb unrest.
If that was his intention, he failed.
Protesters streamed into central Tehran's Revolution Square -- so named for the overthrow of the monarchy 30 years ago -- despite warnings on state television that the rally was illegal and those attending would be viewed as engaging in "incitement" by police who had a right to shoot.
• Read Barbara Slavin's analysis of the unfolding events in Iran: Iran regime likely shaken for good
An Associated Press photographer reported one fatality; otherwise, the demonstration -- after two days of rioting -- was remarkably peaceful.
"For those people who put so much hope and energy and optimism into the political process, I would say to them the world is watching and inspired by their participation regardless of what the ultimate outcome of the election was," President Obama said Monday during an Oval Office meeting with Italian Prime Minister Silvio Berlusconi.
Mr. Obama, in his most detailed reaction yet to the events in Iran, added that many Iranian voters "now feel betrayed" and "the ability of people to peacefully dissent" is a "universal value that needs to be respected," so the world is "rightfully troubled."
The sheer numbers of exuberant demonstrators put police on the defensive. People of every age and social class spread across a five-mile area from Revolution Square to Freedom Square, where a towering concrete monument was erected by Shah Mohammad Reza Pahlavi to celebrate 2,500 years of Persian kings just a few years before he was deposed.
Young men with gelled-back hair and knockoff sunglasses and cell phones walked alongside septuagenarian retirees wearing the Iranian proletariat's trademark cloth trousers and baggy shirts.
Girls sporting green revolutionary chic head scarves and bandannas marched alongside matrons swathed in all-encompassing chadors from which just a single unpowdered nose peeked.
"We want a national referendum, freedom of expression and freedom," said Zahra Hosseini, a 62-year-old homemaker wrapped in a chador who trudged along the route by herself. "There's no going back."
Observers said that the demonstration was the largest spontaneous public event in the history of the Islamic republic. Every Feb. 11, the anniversary of the fall of the Shah's government, thousands of people crowd Freedom Square, but many are bused in by the regime, provided with government-made placards and led by government officials in shouting anti-U.S. and anti-Israel slogans.
On Monday, nothing was staged.
Demonstrators called Mr. Ahmadinejad a liar, offered roses to police officers or walked in silence with green tape over their mouths. At other times, they shouted, "We are Mousavi's Green Army."
Several held aloft signs in Persian and English asking, "Where is my vote?" in a reference to the lower-than-expected vote attributed to Mr. Mousavi and reformist cleric Mehdi Karroubi, who also appeared at the rally. Both men and the third opponent to Mr. Ahmadinejad -- Mohsen Rezaie, a former commander of the Revolutionary Guards -- have demanded an investigation of the vote tally, which gave Mr. Ahmadinejad 63 percent of votes cast.
Other demonstrators called Mr. Ahmadinejad's purported victory a coup d'etat and demanded that his government resign.
Some revived slogans from the Islamic revolution such as Marg bar Zolm (Death to Injustice), which another generation's protesters had chanted at anti-Shah rallies.
"Why should a person who got 24 million votes have to defend his vote with clubbing and riot police?" asked Mansour Gholamali, 46, an office worker. "From the day after the election, all the streets were as if they were under the control of a military government who came to power through a coup d'etat."
"I was around in 1979," said Golzamaneh Rezai, 55, a retiree who struggles to make do on a monthly salary equivalent to about $300. "We had our revolution in order to achieve freedom and once again we're now demanding the same thing: freedom and a share in the oil receipts."
One protester was fatally shot by members of the Basij, a paramilitary group, during a face-off outside a military base close to Freedom Square. Five students were killed Sunday night when their dormitory at Tehran University was attacked. More than 100 students were arrested.
Mr. Obama said Monday that he was "deeply troubled" by the election fraud allegations and "it would be wrong for me to be silent" about the violence and suppression of free speech, but that the United States would continue to pursue "tough, hard-headed diplomacy" with a country that many think is close to developing the capability to make nuclear weapons and has growing influence in the Middle East.
J. Scott Carpenter, a former deputy assistant secretary of state who oversaw the George W. Bush's administration's Iran democracy program, said, "The Obama administration should take advantage of regime weakness to increase its pressure on Tehran both rhetorically, by siding strongly with the Iranian people, and reconsider its decision to mothball the Iran democracy program."
However, former President George H.W. Bush, while expressing skepticism about the official election results, said the U.S. should be prudent in reacting to events in Iran.
"They ought to get to the bottom of it," he said on the debut broadcast of The Washington Times' "America's Morning News." "And without emotion go forward and see if there is something that can be done from outside without inflaming tensions; we don't want to inflame tensions with Iran."
He added that Mr. Obama should refrain from making any military threats.
As cries of "Allahu Akbar" (God is Great) marked the dispersal of the Tehran rally, thick plumes of smoke rose into the darkening sky. Bystanders variously ascribed the smoke to rioting outside a Basij base, the burning of paramilitaries' motorcycles, or the torching of a gas station.
"Since the revolution, we've never seen anything like this," said Hossein, an engineer who gave only his first name. "These people are here to defend their republic."
In keeping with the religious tone of the event, not a single woman removed her head scarf and rules of Islamic morality were maintained throughout.
Ahmadinejad supporters, who staged a rally Sunday, remained clear of the streets.
"Can you compared today's people to yesterday's crowd?" asked Hosseini, the homemaker, in a reference to the Ahmadinejad supporters who had attended the victory rally by the Iranian president. "They bused civil servants, basijis and [Revolutionary Guard] employees in from Pakdasht, Daramin and other distant cities but the people you see today are true Tehranis, not imports."
• Eli Lake and Christina Bellantoni in Washington contributed to this report. Iason Athanasiadis reported from Tehran through a grant from the Pulitzer Center for Crisis Reporting.